Many people in Nevada, Arizona, and California get water and electricity from Lake Mead and Hoover Dam. Shown in Exercise 4 are an x chart (top) and an R chart (bottom) obtained by using the monthly elevations (ft) of Lake Mead at Hoover Dam (based on data from the U.S. Department of the Interior). The control charts are based on the 12 monthly elevations for each of 75 consecutive and recent years. What does the x chart tell us about Lake Mead?